Accidental loadings due to blast or impact may easily cause failure of the elements that are exposed
or located in the vicinity of the hazard, leading in some cases, to the progressive collapse of the
whole structure; therefore, assessment of the structural over strength is critical for structural
engineers to ensure a certain level of security and validate alternative unloading paths. The T-stub
model is used to describe the behaviour of components i) “column flange in bending” and ii) “endplate
in bending” usually present in a beam-to-column bending resistant connection [1]. These
components are responsible for the behaviour in the tension zone of connections, being able to
provide ductility to a connection; thus, proper characterization of T-stub behaviour under impact
loading is crucial.
In this paper, a 3D finite element model exploring the behaviour of a welded T-stub with flange
thickness of 10 mm (T-10) (Fig. 1) is validated against experimental results from:
i) one quasi-static loading (reference case) (grey dotted line Fig. 2 and Fig. 4);
ii) and two rapidly applied dynamic loadings according to the gas pressure in the chamber:
a. 120 Bar (Impact #1 - T10-D120-160 - Fig. 2); and
b. 160 Bar (Impact #1 - T10-D160 - Fig. 4) [2].
The steel grade of the T-stub is S355 and the bolts M20 grade 8.8 are fully threaded. The dynamic
loading simulations take into account the elevated strain rate effects in the stress enhancement,
based on dynamic increase factors, following the Johnson-Cook material model [3]. The dynamic
loadings are applied as a boundary condition in the “pull out surface” (Fig. 1) considering the Tstub’s
transient displacement responses obtained from experimental tests; maximum displacement
values are reached in approximately 0.08 sec. The accuracy of the numerical force-displacement
predictions for both quasi-static and dynamic loading schemes confirms that the Johnson-Cook
material model used, provide accurate stress enhancement to describe the behaviour of bolted steel
connections subject to impact loadings.
From Fig. 2 and Fig. 4, it can be observed that the elastic stiffness remains unchanged for all
loading schemes: ki = 180 kN/m, as the steel’s elastic modulus introduced in the numerical models
are the same for both quasi-static and dynamic situations; moreover, the strain rates developed are
similar for both dynamic loading (Fig. 3), inducing the same dynamic increase factors for the stress
enhancement; the F-δ flows are therefore, similar for both numerical dynamic responses but with
different failure displacements. Plastic resistances of the T-stub: FRd,quasi-static = 161 kN and
FRd,120 Bar = FRd,160 Bar = 195 kN; corresponding to an enhancement of +21% of the plastic resistance
due to the elevated strain rate effects.
Fig. 3 illustrates the pattern of the strain rate (ER), ranging from 1/s to 3/s in the plastic hinge
developed next to the weld toe, corresponding to a DIFs of 1.27 and 1.31. Furthermore, comparison
of the equivalent plastic strain (PEEQ) pattern for both loading situations, shows that two plastic
hinges are developed per flange leg, consistently with the plastic failure mode type 1 predicted by
the Eurocode 3, part 1.8 [1]. However, in the dynamic case, the plastic hinges are slightly
underdeveloped and higher strains are required in the bolt to meet the same deformation level.